variable in filter doesn't work??

when I put a variable in a filter I am having problems...

here is my xml:
-----------------------
<people>
        <person name="george">
                <cat>zoro</cat>
                <dog>butch</dog>
                <fish>jaws</fish>
        </person>
        <person name="jennifer">
                <cat>felix</cat>
                <dog>fido</dog>
                <fish>moby</fish>
        </person>
        <person name="simon">
                <cat>tom</cat>
                <dog>scooby</dog>
                <fish>conroy</fish>
        </person>
</people>
-----------------------

this xsl works properly (it sorts out all except for when cat='zoro':

-----------------------
<xsl:for-each select="//person[cat = 'zoro']">
        <tr>
                <td><xsl:value-of select="cat"/></td>
                <td><xsl:value-of select="dog"/></td>
                <td><xsl:value-of select="fish"/></td>
        </tr>
</xsl:for-each>
-----------------------

output:
zoro   butch   jaws

why doesn't this work?
-----------------------
<xsl:variable name="filter" select='"cat = &apos;zoro&apos;"'/>
<xsl:value-of select="$filter"/>
        <tr>
                <td><xsl:value-of select="cat"/></td>
                <td><xsl:value-of select="dog"/></td>
                <td><xsl:value-of select="fish"/></td>
        </tr>
</xsl:for-each>
-----------------------
output:
zoro   butch   jaws 
felix   fido   moby 
tom   scooby   conroy

I really trying to filter based on a parameter, if the parameter is empty, 
don't filter.  If it is something, filter.

thanks,
will

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